Conventional well-known tin, lead or their alloy plating baths include sulfuric acid baths, alkane sulfonic acid baths, alkanol sulfonic acid baths, borofluoric acid baths, and phenol sulfonic acid baths. They are widely used for plating various electric and electronic parts.
Problems arise when alkane sulfonic, alkanol sulfonic, borofluoric, and phenol sulfonic acid baths are used for plating glass-metal integrated articles containing low-melting sealing glass such as CERDIP and C-QFP type IC packages encapsulated with low-melting glass. These baths can erode the low-melting sealing glass and deposit coatings of tin and solder (tin-lead alloy) on the sealing glass regions with attendant bridging. Like these acidic baths, neutral baths will also erode sealing glass and deposit coatings thereon.
In turn, sulfuric acid baths are advantageous for electrodeposition on glass-metal integrated articles containing low-melting sealing glass because they little erode the sealing glass or deposit coatings thereon. The sulfuric acid bath is applicable to tin plating as a tin sulfate bath, but not to solder plating. Better solder plating is not expected simply by adding lead sulfate to a conventional sulfuric acid bath. It is thus difficult to provide better solder plating to sealing glass-metal integrated articles such as CERDIP and C-QFP type IC packages encapsulated with low-melting glass. Instead of the solder coatings, tin coatings are formed using tin sulfate plating baths. The tin coatings, however, suffer from whisker formation which will cause short circuiting when applied to electric and electronic parts.
In addition, all the sulfuric acid and other baths mentioned above are strong acid baths and have problems in that they can attack the plating apparatus and articles to be plated and in that they generate a strong acid mist during operation, resulting in an undesirable working environment.
There is a need for a plating bath which can plate satisfactory tin, lead, and solder coatings on glass-metal integrated articles containing sealing glass while minimizing corrosion of the plating apparatus and maintaining an acceptable working environment.